A hydraulic excavator performs an excavation operation using a bucket. Such a hydraulic excavator has, in the hydraulic circuit of the hydraulic excavator, a hydraulic circuit for an attachment which allows attachments such as a hydraulic breaker and a crusher to be used. The hydraulic breaker crushes a rock and the like, and the crusher is used to perform an excavation operation. The hydraulic circuit for attachments is provided to operate the attachments.
When the hydraulic breaker or the crusher is installed as a front attachment, a variable relief valve is provided in one of hydraulic lines that connect an actuator for an attachment to a directional control valve for operation of the attachment in order to allow the hydraulic breaker or the crusher to be used. The hydraulic line is a part of the hydraulic circuit for an attachment and is provided on the side of the actuator. Pressure of a hydraulic fluid (for pilot operation) that is to be supplied to a pilot operating section included in the variable relief valve is increased or reduced by a set pressure changing device that includes a switch and a solenoid valve so that set pressure of the variable relief valve can be changed (refer to, for example, Patent Document 1).    Patent Document 1: JP, A 2004-76411 (Paragraphs 0056-0059 FIGS. 6 and 7)